Revisionists have recently challenged a basic plank of modern neuroscience, the neurone doctrine, which asserts that the neurone forms the basic unit of action in the CNS. The challenge stems from circumstantial evidence that different individual neurites from the same neurone can act as independent input-output republics, functonally divorced from each other: 'local neuronal circuits' (LNCs). A conjunction of events make is practicable to attempt the definitive test of this idea. These are: (1) circumstantial evidence that the yield neurites of certain insect visual amacrine neurones may act as independent LNCs; (2) improved knowledge of the anatomy, pointing the way to test yield independence; (3) the development of a single fiber optic mapping technique bythe P.I.: this takes advantage of the inherent optical isolation between lenses in the compound eye, allowing independent stimuli to be delivered tothe separate yield domains; (4) the fact that the P.I. is relocating from 11/81 for less than 2 years next door to the second investigator (Meinertzhagen), whose laboratory has the anatomical/ultrastructural expertise necessary for the project. We propose to record intracellularly from amacrines and dye-mark them, so revealing the distribution and exact number of their yield neurites. In these same cells, the degree of isolation/interaction between yields will have already been revealed, by systematically mapping part of the eye with the single fiber optic. In a second test, yield independence will be assayed similarly by tapping the output of another neurone L4, driven locally by a few yields. If the results support the suggested independence of the yield domains in this well-known invertebrate preparation, it will provide the first unequivocal evidence for the LNC principle. This will set an important precedent for workers in vertebrate preparations as diverse as olfactory bulb, retina and cerebellum, where the idea of LNC action first emerged. Our general concept of the unit of action in the CNS will then have changed dramatically, for a significant category of neurone. On a more trestricted front, the experiments should reveal something of the organization of the insect visual amacrines, particularly their involvement in powerful reciprocal synapses with the photoreceptors. Such synapses are widespread in the CNS, but little understood.